Vibrio cholerae is an environmental Gram-negative bacterium and an epidemic and pandemic human diarrheal pathogen. Rapid evolution of the V. cholerae genome in the environment has resulted in exchange of immunity determinants and acquisition of antibiotic resistance traits. These changes have periodically expanded the pool of susceptible individuals or complicated treatment of disease. Natural competence is the ability of bacteria to take up environmental DNA, and the acquisition of new genes through this process is known as horizontal gene transfer. Recently, investigators have found that V. cholerae becomes naturally competent and is able to incorporate new DNA into its genome when cultured under conditions in which quorum sensing is activated and chitin is present. Because V. cholerae abundance in estuaries sometimes correlates with the presence of zooplankton, and these organisms are a rich source of chitin, their exoskeletons are hypothesized to be the environmental surface on which natural competence is activated and the V. cholerae genome evolves. However, chitin, one of the most abundant polymers found on earth, is a surface component of many organisms including algae, fungi, and insects. We propose that, if a host's microbiota transfers particularly advantageous genetic elements to a colonizing pathogen, even a less abundant or less frequently encountered environmental host may have a great impact on a pathogen's success. Multiple publications have reported isolation of V. cholerae, other enteric pathogens, and human commensal bacteria from house flies. In the vicinity of hospitals and farms, the bacteria recovered from flies are often resistant to multiple antibiotics. Therefore, we hypothesized that the fly might provide a rich environment for acquisition of genes encoding virulence traits and antibiotic resistance. Using the fruit fly Drosophila melanogaster as a model, we recently discovered that V. cholerae forms a dense biofilm on the chitinous lining of an intestinal compartment known as the rectum. Preliminary results suggest that quorum and chitin sensing are operative in this compartment, leading to activation of the genes required for natural competence. Here we propose to examine the structures, signals, and regulatory systems that contribute to V. cholerae colonization of the house fly rectum, to study regulation of natural competence genes in the fly rectum, and then to assess horizontal gene transfer in the fly. These studies may impart relevance to the hundreds of studies of V. cholerae biofilm formation in LB broth, may reveal an environmental niche in which V. cholerae natural competence is activated, and may suggest fly control as a simple method to limit the transfer of new virulence and antibiotic resistance traits to pathogenic V. cholerae.